Solutions of cupric ions and complexing agents have been used to dissolve metal, especially copper and copper alloys. This is desirable, for example, in place of ordinary machining for removing specified amounts of these metals from surfaces of fragile or peculiarly shaped objects. A wide-spread application of this technique is the production of printed electrical circuits. In this application, a resist or mask in the form of the desired circuit is placed over a copper film laminated to a base, and the partially masked copper film is placed in contact with an etchant. The copper surface not covered by the resist is dissolved while the copper covered by the resist remains to form the desired circuit pattern.
One such cupric etchant is the well known, highly acidic cupric chloride etchant dissolved in hydrochloric acid. Another etching solution is disclosed in U.S. Pat. No. 3,231,503. This patent teaches a primary etchant solution of a chlorite such as sodium chlorite in an alkaline solution containing an ammonium salt complexing agent for the metal stripped. The etchant is used at a pH of from 8 to 13, preferably above pH 9. It is disclosed in said patent that the useful life of the etchant can be extended upon exhaustion of the primary oxidant, i.e., the chlorite, by increasing temperature to utilize dissolved copper in the cupric state as a secondary oxidant for further dissolution of copper converting the cupric copper to the cuprous form in the process. Accordingly, at this stage of the etching operation, the etchant solution is a cupric ion-ammoniacal etchant comprising an ammonia-containing solution of cupric chloride as the oxidant having a pH between about 9 and 13. The ammonia is the complexing agent holding both cupric and cuprous ions in solution.
An improved cupric-ion-type etchant is disclosed in U.S. Pat. No. 3,650,958. This etchant essentially comprises cupric ions and a non-fuming complexing agent to maintain said cupric ions and dissolved copper in solution, preferably an amine complexing agent capable of forming a solution soluble copper (II) complex and, preferably, a source of chloride or bromide ions.
Though capable of operating within a broad range of pH, dependent upon the selection of the complexing agent, the etchant is preferably an essentially neutral etchant operating within a pH range of 7 to 8. The etchants treated herein are believed to be an improvement over those of the afore-said U.S. Pat. No. 3,231,503 because they are non-fuming, thereby avoiding noxious fumes and, in addition, have the capacity of operating within the preferred pH range of 7 to 8 at which range they do not attack materials used in the manufacture of printed circuit boards such as resins and the like.
In use of the aforesaid etchants, the copper is dissolved by one mole of the cupric ion oxidizing one mole of elemental copper to form two moles of cuprous ion. For this to occur, it is essential that an ion or compound which forms a stable complex ion with cuprous ion be present in the solution. This complexing action effectively reduces the concentration of free cuprous ion in the solution to a level low enough to avoid precipitation of cuprous oxide or the disproportionation of cuprous ion back into cupric ion and free copper. The latter reaction, the disproportionation, is intrinsic to the well-known equilibrium between copper metal, cupric ion and cuprous ion. The equilibrium is such that it effectively prevents dissolution of copper metal by cupric ion to form cuprous ion unless the free cuprous ion in solution is maintained at a very low concentration. Examples of complexing agents that have been used to perform the desired function are chloride and cyanide ions and ammonia. These materials also are capable of forming stable complex ions with cupric ion.
The dissolution continues until the rate decreases to an unacceptable commercial level due to low cupric ion concentration, saturation with dissolved copper, and insufficient complexing agent. As a result of high concentration of copper, in some instances, copper begins to precipitate from solution in the form of a salt or an oxide or hydroxide of copper. If the copper is left in the etchant, etching would stop and the etching equipment would become clogged by the heavy, somewhat gelatinous precipitate.
The spent etchant, as described above, cannot be readily discarded because of strict code regulations prohibiting the dumping of materials which adversely affect the ecology. The dumping of copper, as an example, is generally prohibited. Moreover, dumping of the spent etchant is also economically undesirable because the etchant contains materials that have intrinsic value. For example, copper dissolved in solution has value as scrap metal or as a raw material for preparation of fresh etchant. The complexing agent for the copper is also of value and it would be highly desirable to recover and/or reuse this material.
Various methods have been proposed for treatment of spent etchant. For example, it has been proposed to vaporize the water and collect the solids. However, this method is uneconomical and the recovered solids have to be further treated to recover their components in useful form. A further method proposed in the prior art for treating spent etchant of the ammonium persulphate type, rather than the type treated by the process disclosed herein, comprises electroplating all copper from solution. This method is generally unacceptable because it has for an object removal of all copper to permit dumping. The cost of removing the last remaining parts of copper from solution is expensive and time consuming. Furthermore, the remaining persulphate may be destroyed to a degree by the process, thereby preventing full utilization of the remaining oxidant.